The invention relates to forming a box shaped frame member using internal hydraulic pressure.
It is known to form the cross-sectional profile of a tubular member by a hydroforming process in which a tubular blank is placed within a die and hydraulic fluid pressure is applied to the interior of the tubular blank to expand the tubular blank into the shape defined by the die cavity. The hydroforming process is particularly useful for transforming a commercially available round tubular shape into an elongated box shape tubular frame member of the type useful in the manufacture of automobile vehicle body and chassis.
The prior art has recognized that the circumferential expansion of the tubular blank is limited to about 2-5% (without significantly thinning the tube) when using readily available commercial grades of tubular steel. Accordingly, the circumferential measure of the tubular blank can be only about 2-5% less than the circumferential measure of the box shape end product so that closing the box shape cavity of the hydroforming dies about the round tubular blank is akin to the proverbial problem of forcing a round peg into a square hole.
Accordingly, it would be desirable to provide an improved hydroforming die and method by which a round tubular blank having a diameter greater than the width of the desired finish product could be forced into the hydroforming die, the die closed, and then hydraulic pressure employed to expand the collapsed tube into the desired box shaped defined by the cavity.
In addition, it would be desirable to provide such a die apparatus of economical manufacture and low maintenance.
According to the invention, a method of forming an elongated box shape tubular frame member from a round tube includes the steps of: providing a lower die member having a cavity submerged substantially below the top surface of the lower die and having a cavity width equal to the width of the desired box shape frame member and less than the diameter of the tube. The lower die has an entry opening which opens to the top surface of the die and is defined by angularly disposed funnel walls which converge to intersect with the cavity. The round tubular blank is placed onto the lower die so that the blank rests on the angularly disposed funnel walls above the cavity. The upper die is lowered onto the lower die and has a lid wall which forces the tubular blank downwardly into the cavity while the angularly disposed funnel walls guide the blank to collapse in a manner preventing pinching of the blank between the upper and lower dies. The application of internal hydraulic pressure then expands the collapsed tube outwardly to the shape defined by the cavity of the lower die and the lid wall of the upper die. The lower die preferably includes rounded corners at the intersection of side walls and a bottom wall to provide rounded corners on the box shape tubular frame member. However, the edge surfaces of the lid wall of the upper die are rounded in the reverse direction away from the side walls of the lower cavity for ease of die manufacture, so that the rounded corners of the box shape tubular frame member are provided by controlling the magnitude of internal hydraulic pressure to a level sufficient to properly expand the tube against the side walls, the bottom wall, the lid wall, and the lower corners, while not so great as to force the upper corners fully into the reverse rounded upper corners of the die cavity.